Types vs Interfaces

Programming in TypeScript offers us the luxury to ensure type checking which leads to effective and error-free coding. Two significant tools in our TypeScript toolbox for enforcing these checks are: Types and Interfaces. On the surface, both look similar and are often used interchangeably. However, they have their unique features and use cases. This document breaks down these differences to help you decide what’s best for your TypeScript projects.

What are Types?

In TypeScript, types provide a way to define a contract for variables, object properties, function return values and function parameters. It’s a definition of what kind of data structure it is, which attributes it has, and more.

type Vehicle = {
  make: string;
  model: string;
};

In the example, Vehicle is a type that expects an object with the properties make and model, both of which are string.

What are Interfaces?

Interfaces, like types, enable you to define the structure and set boundaries for objects and classes. They are powerful constructs that promote robust, clear, and reliable code.

interface Vehicle {
  make: string;
  model: string;
};

In this example, Vehicle is an interface that describes an object structure with make and model properties of the string type.

Differences Between Types and Interfaces

While types and interfaces may initially seem alike, they do have key differences.

Application

An Interface is strictly used for defining the shape of an object, whereas a type has a wider range of applications. A Type can represent primitive types (like number, string, boolean, etc.), unions, intersections, mapped types, conditional types, and more.

type numOrString = number | string; // Union type

Intersection Types (&) vs Extending Interfaces

Types allow the creation of intersection types using the & operator. Similarly, Interfaces can achieve the same result using the extends keyword.

When using a type:

type A = {...}
type B = {...}
type C = A & B;

When using an interface:

interface A {...}
interface B {...}
interface C extends A, B {...}

Interface Merging

Interfaces offer a benefit in that they can be declared multiple times and they’ll be merged together to create the final interface. This is not possible with types.

interface Box {
   height: number;
   width: number;
}

interface Box {
   scale: number;
}

let box: Box = {height: 5, width: 6, scale: 10};

By understanding the unique features of Types and Interfaces, you can make informed decisions about when to use them depending on the specific requirements of your project.

Performance Implications

Even though TypeScript compiles down to JavaScript, where there’s no performance difference between types and interfaces, there can be a small impact on compile times. Generally, interfaces tend to be slightly more efficient, leading to quicker TypeScript compilation times, especially for larger codebases.

Which Should You Use?

While TypeScript’s official guidelines lean towards using interfaces, many in the community find the robust features of types more appealing.

The key takeaway is consistency - it’s better to stick to one over the other in your codebase.

Consider your specific requirements and decide accordingly - interfaces for simplicity and better error management, types for additional functionalities like unions and representing primitive types.